1. Field of the Invention
An aspect of the present invention relates to at least one of a fixing device for fixing a toner image on a sheet-shaped recording medium and an image formation apparatus.
2. Description of the Related Art
In an image formation apparatus for forming an image on a recording paper sheet (paper sheet) that is a sheet-shaped recording medium by means of an electro-photographic method as a copying machine, a facsimile machine, or a printer, a photoconductor body is electrically charged by an electrical charging device and subsequently an electrostatic latent image is formed on a surface of the photoconductor body by optical writing in accordance with an original image or image information, or the like. This electrostatic latent image becomes a toner image that is a visible image, due to a developing device. The recording paper sheet is heated and pressurized by a fixing device so that this toner image is fixed on the recording paper sheet to obtain a copy or a print.
For a fixing device used in an image formation apparatus, two methods are used currently. One is a roller method wherein a heating roller with a heat generation part in an interior thereof and a pressurizing roller that rotates while contacting the heating roller are provided and a recording paper sheet is conveyed to a gap between the heating roller and the pressurizing roller to heat and pressurize the recording paper sheet and thereby fix a toner image thereon. The other is a belt method wherein an endless-type fixing belt extends on a heating roller with a heat generation part and a heating roller and a recording paper sheet is conveyed to a gap between this fixing belt and a pressurizing roller to be heated and pressurized and thereby fix a toner image thereon. Furthermore, a heat source may be placed directly on an inside of the fixing belt.
High productivity or energy saving is desired for an image formation apparatus. Furthermore, an applicability for dealing with recording paper sheets with various thicknesses or sizes is also desired. In particular, in a case where recording paper sheets that are composed of cardboard are continuously conveyed while high productivity is kept, thermal energy stored in a fixing device is removed sequentially. Hence, it is necessary to energize a heat generation source for heating and thereby replenish lost heat in order to ensure a fixation property.
In a fixing device that uses an infrared ray heater as such a heat generation source, it is desired that a rate of lighting of the infrared ray heater is increased to heat a heating roller quickly. Furthermore, in a case of a sheathed heater, it is necessary to increase a rate of energizing thereof. Herein, a rate of lighting or a rate of energizing refers to a rate of an amount of a supplied electric power for providing a practical amount of heat generation to an amount of a supplied electric power corresponding to a maximum capability of heat generation.
In Japanese Examined Patent Application Publication No. 63-044223, a problem in a case where a width of a conveyed paper sheet is small is that wasteful thermal energy is caused, and two heaters in which a lighting heater length necessary for a maximum size of a paper sheet to be conveyed is divided into two are shifted and arranged so that the heaters heat an entirety of a fixing roller uniformly. Thus, energizing is conducted in such a manner that two heaters are energized in a case of a recording medium with a large paper sheet width to be conveyed and only a heater at a side of a paper sheet to be conveyed is energized in a case of a recording medium with a small paper sheet width to be conveyed, so that it is possible to conduct fixation with no wasteful energy and a uniform temperature distribution.
Recently, it is desired that a printing operation is conducted at a higher speed. Accordingly, an amount of heat removed from a fixing roller by a paper sheet is increased so that a larger temperature difference is readily caused between a conveyance part and a non-conveyance part for a paper sheet. This is particularly significant in a fixing device in which a thickness of a fixing roller is decreased to improve a rate of temperature elevation and a heat capacity of the fixing roller is decreased. It is necessary to light a heater at a higher rate of lighting in order to replenish heat removed by a paper sheet. Accordingly, there is a problem that excessive thermal energy is provided at a non-conveyance part for a paper sheet to elevate temperature, and in a worst case, to exceed a heat-resistant temperature of a fixing roller (or belt) and a possibility of causing a serious issue such as deformation of a member is provided.
Furthermore, a low cost while attaining high functionality such as high productivity or energy saving as described above is also an important element. A method for prolonging a replacement cycle of a replacement component to reduce a cost of such a component or a cost of replacement by a serviceman or a method for reducing the number of components is considered as means for attaining cost reduction. However, any of the methods has a possibility of causing a problem in durability or a problem of functional degradation.
An object of an embodiment according to the present invention may be to solve such a problem.